Memory Systems and Methods for Dynamically Phase Adjusting A Write Strobe and Data to Account for Receive-Clock Drift

ABSTRACT

A memory system includes a memory controller that writes data to and reads data from a memory device. A write data strobe accompanying the write data indicates to the memory device when the write data is valid, whereas a read strobe accompanying data from the memory device indicates to the memory controller when the read data is valid. The memory controller adaptively controls the phase of the write data strobe to compensate for timing drift at the memory device. The memory controller uses read signals as a measure of the drift.

FIELD

The subject matter disclosed herein relates generally to circuits forcommunicating data between integrated circuits, and in particular tocircuits and associated methods for phase shifting data and strobesignals to accommodate drift in a clock signal.

BACKGROUND

SDRAM, or synchronous dynamic random access memory, is a type of memoryintegrated circuit that waits for rising or falling edges of a timingreference signal before responding to control inputs. Typical examplesof timing reference signals include clock signals and strobe signals.DDR SDRAM, or double-data-rate SDRAM, achieves greater bandwidth thanordinary SDRAM by transferring data on both the rising and falling edgesof timing reference signals.

Many DDR SDRAMs that produce data also produce a data strobe, called“DQS” (“data query strobe”), to indicate that data is valid. The DQS istransmitted, along with data, from the memory controller to the DDRSDRAM during write operations and from the DDR SDRAM from the memorycontroller during read operations. When driven by the memory controller,DQS is center-aligned with the write data. When driven by the memory,DQS is edge-aligned with the read data.

The timing for write operations is often defined in a specification. Forexample, in at least some DDR SDRAM specifications, the time t_(DQSS)between a write command and the first corresponding rising edge of DQSis specified with a relatively wide range (from 75% to 125% of one clockcycle). The time t_(DQSS) might be described as a window during whichthe specified DDR SDRAM “looks for” data on a data bus. Devices issuinga write command to such a DDR SDRAM are expected to drive DQS in such away that the signal arrives at the DRAM pins at a clock edge, plus orminus 25% of one clock cycle.

Designing a memory controller that provides the write DQS within atiming window t_(DQSS) can be complicated by the fact that the memorycontroller is desired to operate in many different system topologies.For example, relatively short, lightly loaded channels may lead a DQS toarrive too early; whereas relatively long, highly loaded channels maylead a DQS to arrive too late. In either case, the early or late DQS mayviolate the specification requirement for the timing window t_(DQSS),and potentially lead to an error. Other system variations, such as thosethat result from process variations and temperature and supply-voltagefluctuations, also affect signal propagation delays and thereforefurther complicate the task of maintaining the relative timing of theDQS and the write signal within the requisite window.

Meeting the DQS timing window t_(DQSS) can be particularly daunting ifthe memory device does not include clock recovery circuitry to stabilizethe device-side clock signal used to time the write signal. DDR DRAMsadapted for use in mobile devices often lack clock-recovery circuitry,which advantageously reduces standby power and standby-active transitionlatency. Unfortunately, these benefits come at the cost of increasedwrite signal drift, leading to an increased probability of violating thet_(DQSS) timing parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter disclosed is illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

FIG. 1 depicts a memory system 100, including a memory controller 105and at least one memory device 110.

FIG. 2 depicts a flowchart 200 illustrating a process of calibrating andadaptively adjusting memory controller 105 in accordance with oneembodiment.

FIG. 3 depicts an embodiment of memory system 100 of FIG. 1 in moredetail, like-identified elements being the same or similar.

FIG. 4 depicts embodiments of delay circuit 486 and skip circuit 488,which may be used to implement the delay circuit 186 and skip circuit188 of FIGS. 1 and 3.

FIG. 5 is a timing diagram 500 illustrating operation of skip circuit488 and delay circuit 486 of FIG. 4 for the case in which DlyC[2:0] is010 and DlyF[1:0] is 01.

DETAILED DESCRIPTION

FIG. 1 depicts a memory system 100, including a memory controller 105and at least one memory device 110. In accordance with the depictedembodiment, memory controller 105 adaptively controls the timing of thewrite DQS to compensate for timing drift of write signals within memorydevice 110, and therefore to prevent violations of the t_(DQSS) timingwindow. Read DQS signals from memory device 110 provide a measure ofwrite-signal timing. Memory controller 105 monitors the phase of readDQS signals to sense and correct for write-signal drift.

Memory controller 105 includes control logic 115 that issues address andcontrol signals to a command interface 120, conveys byte-widetransmit-data signals TD to a variable-delay write circuit 125, andreceives byte-wide receive-data signals RD from a variable delay readcircuit 130. A distributed clock signal PClk defines the clock domainfor control logic 115, interface 120, and portions of variable-delaywrite and read circuits 125 and 130. Respective write and readphase-reference signals PClkWc and PClkRc, each a phase shifted versionof clock signal PClk in this embodiment, respectively define the writeand read clock domains.

Memory device 110, a strobed DDR DRAM in this example, includes a clockdistribution network 140, a command decoder 145, a write circuit 150,and a read circuit 155, all of which communicate with a DRAM core 160.Memory device 110 additionally includes a plurality of pads 165 coupledto corresponding pads 135 of controller 105. In this example, one ofpads 135 is a shared strobe terminal that both conveys the write strobeto and receives the read strobe from memory device 110, and data linesDQ share collections of pads 135 to transmit and receive data. Separateunidirectional data lines or separate strobe lines can be used fortransmit and receive operations in other embodiments.

Clock signal PClk from control logic 115 times both the memorycontroller and the memory device. The clock path between memorycontroller 105 and the various components of memory device 110,including clock distribution network 140, impose a clock delay, so thedevice-side clock domain is defined by a distributed clock signal Ckbthat may not be phase aligned with controller-side clock signal PClk.Including a clock-recovery circuit in network 140 can ameliorate thisphase misalignment; however, clock recovery circuitry consumes standbypower and increases the time required to activate the memory device, andis therefore undesirable for some applications.

To perform a write operation, control logic 115 issues the appropriateaddress and control signals to command interface 120. The followingdiscussion is limited to write command WCa for ease of illustration, asthe remaining address and control signals will be readily understood bythose of skill in the art. Command interface 120 includes a synchronousstorage element 170 that times write command WCa to clock signal PClk.The resulting signal WCac is then conveyed to command decoder 145 assignal WCad. (The present disclosure employs a naming convention inwhich signals ending with a lower-case “c” are control-side signals, andsignals ending in a lower-case “d” are memory-device-side signals. Thisnaming convention recognizes that signals communicated between memorycontroller 105 and memory device 110 shift in phase.) Synchronousstorage element 175 within command decoder 145 retimes control signalWCad to the device-side time domain defined by signal Ckb to create awrite signal Write to core 160.

During a write operation, variable-delay write circuit 125 issues, on awrite-strobe terminal DQSWc, a like-named, center-aligned write strobewith write data DQ to memory device 110. (In general, signals and theirassociated nodes carry the same designations. Whether a given monikerrefers to a signal or a corresponding node will be clear from thecontext.) The timing of the data and strobe signals is based upon aclock signal PClkWc, a delayed version of clock signal PClk. As detailedbelow, the delay imposed upon clock signal PClkWc is set to ensuresystem 100 meets the t_(DQSS) requirement imposed by the DDR SDRAMspecification.

At memory device 110, write circuit 150 captures the write data DQ frommemory controller 105 using a strobe DQSWd, a delayed version of DQSWc,and retimes the captured data to the memory-device clock domain as dataWRD. As noted previously, per the t_(DQSS) specification the timebetween the write command Write and the first corresponding rising edgeof DQS is specified as from 75% to 125% of one clock cycle. In memorydevice 110, this timing window t_(DQSS) corresponds to the phasedifference between the strobe DQSWd and the edge of clock signal Ckbthat accompanies write command Write.

During a read operation, read circuit 155 issues a read data strobeDQSRd edge-aligned with data DQ to memory controller 105. Variable-delayread circuit 130 then captures the read data DQ using a clock signalPClkRc phase aligned with a delayed version of read strobe DQSRd,controller-side read strobe DQSRc. Variable-delay read circuit 130 thenretimes the captured data to the controller clock domain PClk as dataRD. A phase comparator 180 maintains the alignment between clock signalPClkRc and read strobe DQSRc by occasionally comparing the phases ofthese two signals and phase adjusting clock signal PClkRc as needed toreduce any phase difference. In this embodiment, comparator 180 uses acontrol signal Inc/Dec to adjust the signal-propagation delay through adelay circuit 182 to phase adjust clock signal PClkRc. Control signalInc/Dec is also used in this embodiment to advance or retard the writeclock domain defined by clock signal PClkWc.

In variable-delay write circuit 125, a skip circuit 188 samples datasignals TD from control logic 115 using clock signal PClk and retimesthe captured data to clock signal PClkWc, a delayed version of clocksignal PClk. The delay imposed by delay circuit 186 is set to optimizethe timing of controller-side strobe DQSWc, and thus the device-sidestrobe DQSWd, to maintain the delay between the strobe DQSWd and theedge of clock signal Ckb that accompanies write command Write within thespecified t_(DQSS) window.

FIG. 2 depicts a flowchart 200 illustrating a process of calibrating andadaptively adjusting memory controller 105 in accordance with oneembodiment. In a calibration process 205 that may be performed atinitialization or another time, memory controller 105 initiates a seriesof dummy write operations 210 to memory device 110, each write operationusing different delayed versions of a write data signal. In this step,memory controller 105 may write pre-specified data bytes to memorydevice 110, independent of any data needs of components of the memorysystem or other higher layer machine-readable code; these writes may beperformed at power-up, or other intervals in which the memory componentwas otherwise not being utilized.

Following completion of the dummy write operations 210, controller 105reads the data of all dummy write operations from memory device 110(step 215) and compares the read data with a copy of the originallywritten data to identify successful write operations (step 220). Timinginformation corresponding to the successful dummy write operationsallows for identification of the particular delayed write data signalproviding the best timing margin (step 225). The logic values thatidentify the delayed write data signal providing the best timing marginmay then be programmed into one or both of delay circuits 182 and 186(step 227).

Delay adaptation 230 follows calibration 205 to accommodate phase driftthat occurs over time due to e.g. temperature and supply-voltagefluctuations. Per decision 235, phase comparator 180 in controller 105occasionally compares read strobe DQSRc with clock signal PClkRc duringread operations. If these two signals are out of phase, comparator 180adjusts delay circuit 182 as needed to maintain synchronization betweenthe two signals (step 240). In some embodiments, delay circuit 182 canbe calibrated to introduce a phase offset (e.g., 90 degrees) that isadaptively maintained.

In embodiments where the adjustment is dynamic, a minimum read commanddensity or rate may be needed. In particular, because the adjustmentonly occurs when read data is received by memory controller 105,excessive timing drift may occur if read commands are issuedinfrequently. In some embodiments, therefore, control logic 115 mayissue one or more supplemental read commands to the memory device 110 ifa time interval since a last read command exceeds a predetermined value.

Turning to variable-delay write circuit 125, delay circuit 186 is set atinitialization to maintain the t_(DQSS) parameter within the specifiedwindow as discussed above. Once set, however, the t_(DQSS) window canvary with temperature and supply voltage fluctuations. Clockdistribution network 140, a primary contributor to such variations,allows clock signal Ckb to drift with respect to write strobe DQSWd.Storage element 175 synchronizes the Write command to clock signal Ckb,so the Write command likewise drifts with respect to write strobe DQSWd.If substantial, such drift can cause memory system 100 to violate therequired t_(DQSS) window.

Recall that time t_(DQSS) is the time between the strobe DQSWd and theedge of clock signal Ckb that accompanies write command Write. Becausethe write command Write is timed to clock signal Ckb, clock signal Ckbprovides a measure of write-signal drift. Read strobe signal RQSRd isalso timed to clock signal Ckb, and is therefore also a measure ofwrite-signal drift. In other words, both write command Write and readstrobe RQSRd are timed to clock signal Ckb, and therefore drift togetherwith clock signal Ckb. Memory controller takes advantage of thisrelationship by altering the phase of write strobe DQSWc to account fordrift in the read strobe signal RQSRd, and thus to account for drift inclock signal Ckb that might otherwise induce a violation in thespecified t_(DQSS) window.

Comparator 180 issues control signals Inc/Dec to delay circuit 182 tomaintain phase alignment between clock signal PClkRc and the receivestrobe DQSRc. These phase adjustments accommodate phase changes inreceive strobe DQSRc that are induced by changes in the phase of clocksignal Ckb, and are consequently similar in magnitude to the phasechanges experienced by write command Write. Control signals Inc/Dec arealso conveyed to delay circuit 186 within variable-delay write circuit125 to adjust clock signal PClkWc by the same phase change imposed bydelay circuit 182 to accommodate changes in strobe DQSRc. Write strobeDQSWc is timed to clock signal PClkWc, and so is likewise phase adjustedto accommodate drift in the receive strobe DQSRc, and thus the similardrift in write signal Write.

FIG. 3 depicts an embodiment of memory system 100 of FIG. 1 in moredetail, like-identified elements being the same or similar. In additionto delay 186 and skip circuit 188 of FIG. 1, variable-delay writecircuit 125 includes input registers 302 and 304 timed to clock domainPClk, output registers 306, 308, and 310 timed to the PClkWc domain, astrobe-pattern generator 312, a multiplexer 314, and a pair of outputbuffers 316 and 318. Registers 302 and 304 capture transmit-enablesignal Ten and eight-bit-wide data TD, respectively, upon rising edgesof clock signal PClk. Skip circuit 188 retimes the outputs of registers302 and 304 to the PClkWc domain and provides the retimedtransmit-enable signal TenF to register 306 and four-bits of data TD toeach of registers 308 and 310 for capture by respective rising andfalling edges of clock signal PClk. Multiplexer 314 then alternatelytransmits the contents of registers 308 and 310 via driver 318 to memorydevice 110. Each time multiplexer 314 issues a burst of parallel databits, pattern generator 312 transmits a write-strobe pattern (e.g.,1010) DQSWc to accompany the data to memory device 110. Both thetransmitted strobe DQSWc and the write data DQ are timed to PClkWc, thephase of which can be adjusted relative to clock signal PClk byasserting a load signal Ldt to capture an applied transmitphase-adjustment signal Tpht.

Variable-delay read circuit 130 may function in substantially the samemanner as variable-delay write circuit 125, so a detailed discussion ofvariable-delay read circuit 130 is omitted for brevity. In summary,variable-delay read circuit 130 transfers read data DQ from clock domainPClkRc to clock domain PClk in the presence of an assertedreceive-enable signal Ren. An AND gate 320 issues an enable signal ENthat allows comparator 180 to update the delay calibration for delayelements 182 and 186 in the presence of a version of the read-enablesignal, Renc, retimed into the PClkRc clock domain. The timing of clocksignal PClkRc can be adjusted by asserting a load signal Ldr to capturean applied receive phase-adjustment signal Tphr.

In an alternative embodiment (not shown), variable-delay read circuit130 may include a FIFO block for receiving the read data, with theoutput of gate 320 causing the read data to be loaded into the FIFO. Theread data may then be unloaded from the FIFO as receive-data signals RD.Though clock signal PClkRc is not used to sample read data in thisalternative embodiment, comparator 180 and delay element 182 may stillbe included for updating the write delay.

FIG. 4 depicts embodiments of delay circuit 486 and skip circuit 488,which may be used as delay circuit 186 and skip circuit 188 of FIGS. 1and 3. For simplicity, the discussion of skip circuit 488 is limited tothe phase shifting of transmit-enable signal Ten: variable-delay writecircuit 125 similarly retimes other signals between the PClk and PClkWcdomains.

Delay circuit 486 includes an offset clock generator 405, a multiplexer410, and an n-bit counter 415. Skip circuit 488 includes collections oflatches 420 and 425 and a pair of multiplexers 430 and 435. Register 306from FIG. 3 is also included in FIG. 4 to show how the respectiveoutputs TenF and PClkWc of skip circuit 488 and delay circuit 486 may beused to produce a transmit enable signal TenFc retimed to the PClkWcdomain.

Counter 415, a 5-bit counter in the depicted embodiment, is loaded witha calibrating value at initialization, as detailed above in connectionwith FIG. 2. The five bits loaded into counter 405 are presented on itsoutput as course delay bits DlyC[2:0] and fine delay bits DlyF[1:0].These delay signals together determine the delays imposed on thetransmit-enable signal Ten and clock signal PClk to obtain the adjustedenable signal TenFc and write clock signal PClkWc. These signals canthen be phase adjusted as needed by incrementing or decrementing counter415 as described above in connection with FIGS. 1 and 2.

FIG. 5 is a timing diagram 500 illustrating operation of skip circuit488 and delay circuit 486 of FIG. 4 for the case in which DlyC[2:0] is010 and DlyF[1:0] is 01. DlyC[2:0] is coupled to the select port ofmultiplexer 430, and causes multiplexer 430 to select its #2 (01) inputto provide output signal TenC, a version of transmit enable signal Tendelayed by three clock cycles via latches 420. Signal TenC can thus bedelayed in increments of one PClk cycle by appropriate selection ofDlyC[2:0] values. Latches 425 then present the coarsely adjusted enablesignal TenC to the inputs of multiplexer 435 on both the rising andfalling edges of clock signal PClk. Because DlyF[1] is a zero in thisexample, multiplexer 435 selects the output of the one of latches 425with an inverted clock input. The output TenF from multiplexer 435therefore takes the value of TenC following the next falling edge ofclock signal PClk.

Offset clock generator 405 provides four clock signals PClk00, PClk01,PClk10, and PClk11 having different phase offsets relative to the clocksignal PClk. PClk00, PClk01, PClk10, and PClk11 are delayed with respectto the PClk signal by 0, 0.25, 0.5, and 0.75 clock cycles, respectively.Multiplexer 410 then selects one of these four clock signals based uponthe value of fine delay signal DlyF[1:0] to produce the phase-shiftedwrite clock signal PClkWc. DlyF[1:0] is 01 in the present example, somultiplexer 410 outputs signal PClk01 as write clock signal PClkWc.Register 306 captures the adjusted transmit-enable signal TenF on therising edge of clock signal PClkWc to produce the final transmit enablesignal TenFc phase adjusted by 0.25 clock cycles into the PClkWc domain.TenFc and PClkWc can be similarly phase adjusted 0, 0.5, and 0.75 bysetting fine delay signal DlyF[1:0] to 00, 10, and 11, respectively.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols are set forth to provide a thoroughunderstanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, the interconnection betweencircuit elements or circuit blocks may be shown or described asmulti-conductor or single conductor signal lines. Each of themulti-conductor signal lines may alternatively be single-conductorsignal lines, and each of the single-conductor signal lines mayalternatively be multi-conductor signal lines. Signals and signalingpaths shown or described as being single-ended may also be differential,and vice-versa. Similarly, signals described or depicted as havingactive-high or active-low logic levels may have opposite logic levels inalternative embodiments. As another example, circuits described ordepicted as including metal oxide semiconductor (MOS) transistors mayalternatively be implemented using bipolar technology or any othertechnology in which a signal-controlled current flow may be achieved.With respect to terminology, a signal is said to be “asserted” when thesignal is driven to a low or high logic state (or charged to a highlogic state or discharged to a low logic state) to indicate a particularcondition. Conversely, a signal is said to be “de-asserted” to indicatethat the signal is driven (or charged or discharged) to a state otherthan the asserted state (including a high or low logic state, or thefloating state that may occur when the signal driving circuit istransitioned to a high impedance condition, such as an open drain oropen collector condition). A signal driving circuit is said to “output”a signal to a signal receiving circuit when the signal driving circuitasserts (or de-asserts, if explicitly stated or indicated by context)the signal on a signal line coupled between the signal driving andsignal receiving circuits.

An output of a process for designing an integrated circuit, or a portionof an integrated circuit, comprising one or more of the circuitsdescribed herein may be a computer-readable medium such as, for example,a magnetic tape or an optical or magnetic disk. The computer-readablemedium may be encoded with data structures or other informationdescribing circuitry that may be physically instantiated as anintegrated circuit or portion of an integrated circuit. Although variousformats may be used for such encoding, these data structures arecommonly written in Caltech Intermediate Format (CIF), Calma GDS IIStream Format (GDSII), or Electronic Design Interchange Format (EDIF).Those of skill in the art of integrated circuit design can develop suchdata structures from schematic diagrams of the type detailed above andthe corresponding descriptions and encode the data structures oncomputer readable medium. Those of skill in the art of integratedcircuit fabrication can use such encoded data to fabricate integratedcircuits comprising one or more of the circuits described herein.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, the skip and delaycircuitry disclosed herein are examples, but are not limiting. Manyother circuits and methods for crossing clock domains are well known tothose of skill in the art. Embodiments of the invention may be adaptedfor use with multi-pulse-amplitude-modulated (multi-PAM) signals.Moreover, some components are shown directly connected to one anotherwhile others are shown connected via intermediate components. In eachinstance the method of interconnection, or “coupling,” establishes somedesired electrical communication between two or more circuit nodes, orterminals. Such coupling may often be accomplished using a number ofcircuit configurations, as will be understood by those of skill in theart. Therefore, the spirit and scope of the appended claims should notbe limited to the foregoing description. Only those claims specificallyreciting “means for” or “step for” should be construed in the mannerrequired under the sixth paragraph of 35 U.S.C. § 112.

1. A memory controller to control a memory device, the memory controllercomprising: a write circuit operable to transmit at least one of awrite-timing reference signal and a write-data signal having awrite-signal phase; a read circuit operable to receive a read signalhaving a read-signal phase from the memory device; and a phasecomparator coupled to the write circuit and operable to control thewrite-signal phase responsive to the read-signal phase.
 2. The memorycontroller of claim 1, wherein the write circuit transmits thewrite-timing reference signal and the write-data signal, and maintains afixed phase relationship between the write-timing reference signal andthe write-data signal.
 3. The memory controller of claim 2, wherein thewrite-timing reference signal is a strobe signal.
 4. The memorycontroller of claim 1, wherein the phase comparator is coupled to theread circuit and operable to control a read-clock phase responsive tothe read-signal phase.
 5. The memory controller of claim 1, wherein thewrite circuit is operable to time the write-data signal to thewrite-timing reference signal.
 6. The memory controller of claim 5,wherein the write circuit further comprises a skip circuit to retime thewrite-data signal from a first clock domain to a second clock domainestablished by the write-timing reference signal.
 7. The memorycontroller of claim 6, wherein the read signal includes a read-timingreference signal and a read-data signal, wherein the read circuitfurther comprises a second skip circuit to retime the read-data signalfrom a third domain established by the read-timing reference signal tothe first clock domain.
 8. A memory system comprising: a memory deviceoperable to receive at least one of a write-timing reference signal anda write-data signal having a write-signal phase and to convey a readsignal of a read-signal phase; and a memory controller coupled to thememory device and operable to transmit the at least one of thewrite-timing reference signal and the write-data signal to the memorydevice and to receive the read signal from the memory device; whereinthe memory controller adjusts the write-signal phase responsive to theread-signal phase.
 9. The memory system of claim 8, wherein at least oneof the write-timing reference signal and the read signal is a strobe.10. The memory system of claim 8, wherein the read signal includes readdata and a read strobe, and wherein the memory controller furtherincludes a read circuit operable to sample the read data and including adelay circuit operable to delay the read strobe to provide a readreference signal for the sampling of the read data.
 11. The memorysystem of claim 10, wherein the memory controller further including awrite circuit operable to transmit the at least one of the write-timingreference signal and the write-data signal to the memory device, thewrite circuit having a clock terminal receiving a clock signal and asecond delay circuit delaying the clock signal to provide a writereference signal.
 12. The memory system of claim 11, further comprisinga phase comparator coupled to at least one of the delay circuits toadjust the respective read reference signal or write reference signal.13. A method for synchronizing data transfers between a memorycontroller and a memory device, the method comprising: issuing, from thememory controller to the memory device, a signal of a write phase;receiving, at the memory controller from the memory device, a readsignal of a read phase; and adjusting the write phase responsive to theread phase.
 14. The method of claim 13, further comprising monitoringthe read phase by comparing the read signal to a clock signal.
 15. Themethod of claim 14, further comprising phase adjusting the clock signalin response to changes in the read phase.
 16. The method of claim 13,further comprising issuing a write command to the memory device,synchronizing the write command to a reference clock signal at thememory device, and synchronizing the read signal to the reference clocksignal at the memory device.
 17. A memory system comprising: a memorydevice having means for receiving at least one of a write-timingreference signal and a write-data signal having a write phase andissuing a read signal having a read phase; and a memory controllerhaving: means for issuing the at least one of the write-timing referencesignal and the write-data signal to the memory device and receiving theread signal from the memory device; and means for phase adjusting the atleast one of the write-timing reference signal and the write-data signalresponsive to phase changes in the read signal.
 18. The memory system ofclaim 17, wherein the means for phase adjusting comprises a phasecomparator for comparing the read signal to a reference clock signal.19. The memory system of claim 17, wherein at least one of the readsignal and the write-timing reference signal is a strobe signal.
 20. Thememory system of claim 17, the memory controller further having areceive clock having a receive-clock phase and means for phase adjustingthe receive-clock phase responsive to the phase changes in the readsignal.